Holographic techniques for storing images are well known. Such techniques are commonly used to store images in a wide variety of different applications. Additionally, various methodologies for utilizing such holographic techniques to store digital data for use in computer systems are currently being explored.
The technique for forming holograms comprises splitting the highly coherent output beam of a laser into separate reference and object beams. The reference beam is directed onto the holographic storage medium, e.g., a photorefractive material, while the object beam is directed onto the object whose image is to be stored. Light from the object is directed to the photorefractive medium wherein an interference pattern is formed due to the interaction of the reference beam with the object beam.
When utilized in digital data storage applications, the object beam typically passes through a spatial light modulator, e.g., a liquid crystal shutter matrix, rather than being reflected off an object, in order to form the holographic image. The spatial light modulator adds the desired digital data to the object beam to facilitate storage of the digital data in the hologram formed therefrom.
Regardless of the application (i.e., the storage of images or data), subsequently,dtirecting a reference beam onto the holographic storage medium results in the reconstruction of an image representative of the originally illuminated object or stored digital data.
Also known are techniques for storing a plurality of such images within a single photorefractive medium via angle-multiplexing of the reference beam. Such angle-multiplexing is discussed in "THEORY OF OPTICAL INFORMATION STORAGE IN SOLIDS", Applied Optics, Vol. 2, No. 4, pg. 393 (1963). The method of angle-multiplexing generally involves maintaining a constant angle for the object beam, while varying the angle of the reference beam for each sequential exposure, i.e., the formation of each separate hologram. Angle-multiplexing thus allows a large number of holograms to be stored within a common volume of photorefractive medium, thereby greatly enhancing the storage density thereof.
Also known are techniques for storing a plurality of such holograms within a spinning drum or disk shaped photorefractive medium. Examples of some holographic memories which utilize drum or disk shaped medium are provided in U.S. Pat. Nos. 3,610,722; 3,737,878; 3,848,096; 4,104,489; 4,224,480; 4,420,829; 4,449,785; 4,929,823; 5,111,445; 5,128,693; 5,285,438; 5,339,305.
However, one problem commonly associated with such contemporary disk and drum based holographic memories is that the geometry of the system is not optimized with respect to the crystalline structure of the storage medium. Further, such contemporary systems do not utilize effective path-length monitoring so as to assure the integrity of holograms within the medium and to assure reliable read-out of a plurality of different sets of angle-multiplexed holograms.
As such, although the prior art has recognized to a limited extend the problem of storing volume holograms in a spinning disk medium, the proposed solutions, to date, have been ineffective in providing a satisfactory remedy.